This invention relates generally to the drying of webs and, more particularly, to methods and apparatus for use in multi-cylinder drying sections of paper machines or the like.
Specifically, the present invention relates to methods and apparatus for ventilating "pockets" in multi-cylinder dryers of paper machines which include two lines of drying cylinders and upper and lower drying wires guided by corresponding upper and lower guide rolls. The paper web is passed over short open draws from cylinders in one line to cylinders in the other.
The running speeds of paper machines have been steadily increasing and presently are approaching speeds of up to about 1500 meters per minute. At such high speeds, fluttering of the web becomes a serious problem hampering the running quality of the paper machine. Although the transfer of the web from the paper machine press section to the drying section, and its subsequent support in areas of single-wire draws are able to be controlled by conventional methods and apparatus, such as those disclosed in prior patents assigned to the assignee of the instant application, difficulties are still encountered at high running speeds in areas of twin-wire draws, such as in the third and fourth drive groups of drying apparatus.
As used herein, the term "single-wire draw" means a conventional arrangement for drawing a web over heated drying cylinders in which the web runs from one line ,of cylinders to the other while supported by a single drying wire. The web is situated between the drying wire and the cylinder surface as it travels over the cylinders of one of the lines. However, on the cylinders of the other line, the web is situated on the outer surface of the drying wire, i.e., the drying wire is situated between the surface of the cylinder and the web. Single-wire draws of this type are advantageous in that the web is supported at all times by the drying wire and has no open draw of any meaningful length, thereby reducing the risk of the formation of wrinkles in the web, as well as the possibility of web breakage.
As used herein, the term "twin-wire draw" refers to a conventional arrangement for supporting and drawing a web over heated drying cylinders in which upper and lower wires are guided by surfaces of the upper and lower cylinders respectively, as well as by upper and lower guide rolls situated in gaps between the upper and lower cylinders. The web is pressed by the upper wire into direct drying contact with the surfaces of the cylinders of the upper line, and, in a corresponding manner, the web is pressed by the lower wire directly against the surface of the cylinders of the lower line.
The web usually has substantially long open draws as it runs from one cylinder line to the other in twin-wire draw arrangements. The web is susceptible to fluttering or breakage as it travels over these open draws, and this problem has become more serious with increased running speeds of paper machines. The problem has become especially serious in the initial part of the drying section where the web is still relatively moist and, therefore, of low strength, and where its elastic properties promote fluttering. Attempts have been made to overcome these problems by shortening the open draws of the web in the initial parts of the drying section by positioning the drying cylinder lines closer to each other, i.e. by situating the imaginary planes passing through the axes of the upper and lower cylinder lines closer to each other than what has been customary, or from that which would be optimal, for example, in view of drying efficiency.
Attempts have also been made to solve the problems created in the running quality in the drying section resulting from increased running speeds of the paper machine by converting the drying section to single-wire draws throughout. However, since single-wire draws have lower drying capacities in view of the fact that the web is carried on the outside of the drying wire as it travels around the lower cylinders, such a solution is generally not advisable except where necessary, for example, in the first and second drying groups of the drying section. Therefore, it is still desirable to operate the subsequent groups in a drying section with twin-wire draws which are, as noted above, preferable to single-wire draws when considered from the viewpoint of drying capacity.
Some conventional solutions for overcoming the problems of web fluttering and breakage over open draws are suggested in U.S. Pat. Nos. 3,751,822 and 3,753,298. These patents disclose drying sections of paper machines provided with twin-wire draws that have been modified by means of arranging the relative positions of the guide and drying cylinders such that the web is supported by a wire substantially at all times and does not have any substantial free or open draws. The ventilation of the spaces or "pockets" formed between the wires and the free or open sectors of the drying cylinders is desirable in such arrangements to reduce fluttering, wrinkles, and breakage of the web caused by air flows induced by the moving wires. In those arrangements disclosed in U.S. Pat. Nos. 3,751,822 and 3,753,298, such pocket ventilation is provided by means of holes or apertures provided in some of the guide rolls through which air is pumped into the pockets.